The present invention relates to a method of electron beam exposure and, more specifically, relates to a method for improving the throughput of a precision pattern generation using electron beam exposure.
The patterning of resist films is essential in the manufacturing of semiconductor devices. That is, many resist films are used as the masks in various etching processes, for example, the fabrication of windows in an insulating layer on a semiconductor substrate, which are used for defining the selective diffusion regions, or the fabrication of fine metallic wiring lines on a semiconductor device.
Such patterning has usually been performed by exposing the resist film to ultra-violet (UV) light passed through a photo-mask, therefore, such technology is referred to as photolithography. However, the recent requirement for the fine patterning on the order of sub-microns in semiconductor devices makes electron beam (EB) technology (referred to as EB lithography) indispensable. The EB permits direct writing of a number of patterns on the order of a micron or sub-micron in a resist film formed on a semiconductor wafer, or in a resist film formed on a glass substrate to be used as a photo-mask in the ordinary photolithographic process.
The most popular of the EB lithography techniques is that an electron beam converged to micron or sub-micron size is scanned over each predetermined region of a resist film, in which a desired pattern is generated. There are two types of EB scanning: a raster scanning of a continuous EB as thin as 0.5 micron, for example, and a stepwise scanning of an EB having relatively wide cross-section, usually rectangular beam on the order of a few microns, wherein the EB is intermittently energized to expose each fraction of the region in a step-and-repeat mode.
FIG. 1 is an enlargement of a partial plan view of an exemplary pattern layout in a photo-mask. Each hatched area constitutes an island-like pattern of an opaque material such as metallic chromium film formed on a glass substrate 1. As one of the smallest dimensions in FIG. 1, the pattern 2, for example, has a linear portion of width "d" of 10 microns or less between ends 21 and 22. The images of these patterns are projected on the resist film formed on a silicon wafer in a demagnification factor of x1/5, for instance. Hence, the actual pattern having the minimum width of 2 micron corresponding to "d" can be formed on the wafer.
FIG. 2 is a conceptual diagram for explaining a method for writing an L-shaped pattern in FIG. 1 by scanning with intermittent shots of an electron beam. Such a complex pattern as the L-shape is usually partitioned into imaginary rectangular patterns, and each rectangular pattern is written by the intermittent shots of an electron beam scanned over each rectangular region in a step-and-repeat mode. That is, in FIG. 2, the region occupied by the L-shape pattern is partitioned into the rectangular regions 3 and 4, each comprising a plurality of respective sub-regions 31 or 41. Each of the sub-regions 31 or 41 has a size determined by the respective equal-length fractions of the sides of the region 3 or 4, and is subjected to the exposure of a single shot of an electron beam having a cross-section of the same size as that of the sub-region.
In FIG. 2, the initial shot of an electron beam is applied to the sub-region 310, which is selected as the origin to define the rectangular region 3, and the successive shots are stepwisely scanned over the region 3 along the arrow-headed lines. The scanning of the shots is usually (but not necessarily) performed along the longer side of the rectangular region except in the transition to the next scanning line. After the completion of the exposure of all sub-regions 31 to the shots, the electron beam is aligned with the sub-region 410, which is selected as the origin to be subjected to the initial shot to the region 4. And then, the intermittent shots of the EB begin to be stepwisely scanned over the region 4 along the arrow-headed lines. Thus, all sub-regions 31 of the region 3 and all sub-regions 41 of the region 4 are subjected to exposure of the intermittent shots of the electron beam, and the L-shape pattern is generated in the resist film.
However, it has been observed that a pattern generated in a resist film often spreads beyond the designed area, as shown in FIG. 3, which illustrates a rectangular pattern 5 (area surrounded by the solid line). The pattern 5 has been generated in the manner as described with reference to FIG. 2. In FIG. 3, the initial shot of an electron beam was applied to the sub-region 510 (hatched area), and subsequent shots were applied to each sub-region 51 (area defined by the dotted lines) along the arrow-headed lines. This spreading increases stepwisely in the sub-regions corresponding to a few shots (2 or 3 shots) after the initial shot to the sub-region 510 and becomes constant at the subsequent sub-region, for example, the sub-region 511. In FIG. 3, if the designed size of the sub-region 51 is 3 microns square, for example, then the maximum spreading may be 0.2 micron at each side. Accordingly, the region 5 (resulting pattern) has an error of 0.4 micron relative to its designed size.
The spreading is more often observed in the resist film formed on a glass substrate and less often observed in the resist film formed on a silicon wafer. And, as mentioned above, the spreading increases within the two or three sub-regions neighboring the starting sub-region at which exposure by the shots has been initiated, and becomes constant in the subsequent sub-regions. Moreover, the spreading is not observed if a sufficiently lengthy pause (blanking period) is provided between the successive shots.
The above facts suggest that the spreading can be caused by a residual effect relating to the shots to the proceeding sub-regions, however, it is not yet clear what causes the residual effect, whether the residual effect affects the shot size of the electron beam or not, and what is the range of the sub-regions subjected to the residual effect of the shot for the proceeding sub-regions. The residual effect also seems to occur in reaction to an attempt to compensate for the effect as it accumulates to a certain degree.
The above-mentioned error of 0.4 micron in the size of the resulting pattern is comparatively large for a master photo-mask (reticle) and decreases the production yield of the relevant semiconductor devices. This is considered to be due to the non-linear interference effect of the optical patterns on the order of a micron or sub-microns, caused by the demagnifying process of the reticle pattern such as shown in FIG. 1.
Such spreading in the resulting pattern could be achieved by controlling the size or dosage of the EB shot to each sub-region, resulting in increased program steps of the control system and decreased throughput.
The EB shot-size- or dosage-controlling involves: (1) the necessity of checking the varied shot size or EB dosage together with the accompanying shot size change, and providing feedback of the checking results to the control system; and (2) the necessity of analog voltage control according to every step of the shot size or dosage checking. Such controlling suffers from poor tolerance for the process condition variables relating to the characteristics of the resist film and the substrate, for example, the material, thickness, sensitivity, etc. of the resist film and the thermal and electric conductivities of the substrate.
Therefore, it is preferable to use an EB shot of constant size and dosage in order to simplify the process control. As mentioned above, the spreading can be avoided by providing a sufficiently lengthy pause (blanking time) between every successive EB shot, but there is a trade-off between the pattern accuracy and the throughput in the pattern generation.